Atherosclerotic coronary heart disease (CHD) represents the major cause for death and cardiovascular morbidity in the western world. Risk factors for atherosclerotic coronary heart disease include hypertension, diabetes mellitus, family history, male gender, cigarette smoke, high serum cholesterol, high low density lipoprotein (LDL) cholesterol levels and low high density lipoprotein (HDL) cholesterol levels. In general, a total cholesterol level in excess of about 225-250 mg/dl is associated with significant elevation of risk of CHD.
A variety of clinical studies have demonstrated that elevated levels of total cholesterol or LDL cholesterol promote human atherosclerosis. Epidemiologic investigations have established that cardiovascular morbidity and mortality vary directly with the level of total cholesterol and LDL cholesterol.
One method for lowering LDL cholesterol levels is by administration of HMG-CoA reductase inhibiting drugs. These drugs antagonize HMG-CoA reductase and cholesterol synthesis in the liver and increase the number of hepatic LDL receptors on the cell-surface to enhance uptake and catabolism of LDL. A drawback of such an approach is that these drugs commonly suffer from a disadvantageous side-effect profile, including, for example, liver toxicity. An alternate approach is to modulate the LDL receptor pathway directly.
PCSK9 (proprotein convertase subtilisin/kexin type 9) is a serine protease family member that binds to and regulates LDL receptor expression on the surface of cells. Inhibition of the LDL receptor-PCSK9 interaction is an attractive approach to the treatment of cholesterol disorders. Inhibition of interactions between large proteins (i.e., protein-protein interactions or PPI) by the use of antibodies or small molecule inhibitors is, however, generally regarded as being particularly difficult and challenging. Large proteins such as PCSK9, with a molecular weight of about 74 KDa, and LDLR, with a molecular weight of about 160 KDa (glycosylated on cell surface; 115 KDa in immature form), are likely to exhibit extensive intermolecular contacts over a large area. The existence of extensive contacts makes it unlikely that a given antibody or small molecule inhibitor will successfully block their binding. Nevertheless, new agents that inhibit the activity of PCSK9 would be useful and the development of such agents would be the product of considerable efforts to overcome technical challenges.
Extracellular matrix proteins, such as matrilins, would appear to be an unlikely place in which to find such an inhibitor. Matrilins are a family of extracellular matrix proteins that, generally, show a similar structure including one or two von Willebrand factor A (vWFA) domains, a varying number of epidermal growth factor (EGF)-like repeats, and a C-terminal coiled-coil domain.
The functions of matrilins have been, heretofore, poorly defined. Matrilins have been thought to play a role in stabilizing the extracellular matrix structure, since they can self-associate into supramolecular structures, resulting in the formation of filamentous networks. It has been shown that at least in the case of matrilin-1 and matrilin-3, these networks can either be associated with collagen fibrils or be collagen-independent. Members of the matrilin family are found in a wide variety of extracellular matrices. Matrilin-1, formerly called cartilage matrix protein, and matrilin-3 are abundant in cartilage, while matrilin-2 and matrilin-4 show a broader tissue distribution.